sound: ask libcanberra to cache alert/desktop switch sounds

These sounds are good candidates for caching in the sound server, to
save a bit of CPU and make reaction faster. Hence, tell libcanberra to
cache them.

https://bugzilla.gnome.org/show_bug.cgi?id=609585

src/core/workspace.c

@@ -494,6 +494,7 @@ workspace_switch_sound(MetaWorkspace *from,
   ca_context_play(ca_gtk_context_get(), 1,
                   CA_PROP_EVENT_ID, e,
                   CA_PROP_EVENT_DESCRIPTION, "Desktop switched",
+                  CA_PROP_CANBERRA_CACHE_CONTROL, "permanent",
                   NULL);
 
  finish: